Thermodynamic Analysis of Rare Earth Oxide Inclusion Modification in Medium-to-High Carbon Steel Weld Overlay Metals

1. Definition and Fundamental Principles

The thermodynamic analysis of rare earth oxide (REO) inclusion modification in medium-to-high carbon steel weld overlay metals addresses the fundamental metallurgical challenge of controlling non-metallic inclusions within weld deposits. In medium-to-high carbon steels (typically 0.40–0.80 wt% C), the weld metal is inherently susceptible to the formation of detrimental inclusions—primarily MnS, SiO₂, Al₂O₃, and various calcium silicates—that degrade toughness, promote crack initiation, and compromise service life in aggressive environments.

Rare earth oxides (principally La₂O₃, CeO₂, Nd₂O₃, and mixed rare earth compositions) function as inclusion modifiers by altering the thermodynamic equilibrium of the molten weld pool. The core thermodynamic principle is governed by the Gibbs free energy of formation (ΔG°) of competing oxide phases. Rare earth elements exhibit exceptionally low solubility in liquid iron and possess a very strong affinity for oxygen, sulfur, and phosphorus. The standard Gibbs free energy of formation for rare earth oxides is significantly more negative than that of FeO or MnO:

• ΔG°(La₂O₃) ≈ −1,124 kJ/mol (at 1873 K)
• ΔG°(CeO₂) ≈ −1,075 kJ/mol (at 1873 K)
• ΔG°(FeO) ≈ −254 kJ/mol (at 1873 K)
• ΔG°(MnO) ≈ −365 kJ/mol (at 1873 K)

This thermodynamic driving force ensures that rare earth oxides preferentially nucleate and grow, scavenging sulfur and oxygen from the melt. The modified inclusions transition from elongated, stringer-like MnS phases to spherical, dispersed REO-based compounds. The modified inclusions exhibit a glassy or mullite-type matrix that bonds cohesively with the ferritic-pearlitic weld microstructure, eliminating the crack-prone MnS/ferrite interfaces.

2. Category and Business Positioning

This research entry belongs to the Metallurgical R&D and Process Optimization category within Cladding Technology Shanxi Co., Ltd.'s capability portfolio. It bridges the gap between fundamental materials science and applied weld overlay engineering, serving as an intellectual property foundation for the company's proprietary consumable development programs.

Within the company's three primary technology routes, this thermodynamic knowledge base is most directly leveraged in:

• TIG/MIG Weld Overlay: Consumable wire and flux formulation optimization for overlay welds on medium-to-high carbon substrates
• Hydraulic Explosive Bonding: Understanding interfacial metallurgical behavior in clad plates where the base material is a medium-carbon steel
• Explosion Welding: Predicting and controlling interfacial reaction products and inclusion distributions in dynamically bonded clad structures

Strategically, this capability positions the company as a technically differentiated provider—capable of offering metallurgically optimized overlay solutions rather than merely executing standard welding procedures. It supports the development of proprietary welding consumables and process parameters that deliver superior performance in demanding applications.

3. Technical Purpose and Value

3.1 Primary Technical Objectives

1. Inclusion Morphology Control: Transform elongated MnS inclusions into equiaxed, dispersed rare earth oxide inclusions that do not act as crack initiation sites
2. Toughness Enhancement: Improve Charpy impact energy and fatigue resistance of overlay welds deposited on medium-to-high carbon steel substrates
3. Crack Resistance Improvement: Reduce hot cracking and cold cracking susceptibility in weld deposits containing elevated carbon and sulfur levels
4. Corrosion Resistance: Eliminate galvanic couples formed between MnS stringers and the surrounding matrix, which are preferential sites for localized corrosion
5. Machinability Retention: Maintain controlled machinability in overlay deposits where REO-modified inclusions provide consistent chip-breaking behavior

3.2 Quantifiable Performance Benefits

Performance Metric	Conventional (No REO)	REO-Modified	Improvement Factor
Charpy V-Notch Impact (−40°C)	15–25 J	45–80 J	2.0–3.2×
Hot Cracking Susceptibility (HCB Test)	60–85% Cracking	10–25% Cracking	3.0–5.0× reduction
Inclusion Elongation Ratio (AEN)	15–35	1–4	5.0–10.0× improvement
Corrosion Rate (5% H₂SO₄, 24h)	0.8–1.5 mm/y	0.3–0.6 mm/y	2.5–3.0× reduction
Weld Metal Sulfur Content	0.015–0.035%	0.003–0.010%	3.0–5.0× reduction

3.3 Strategic Value to the Company

The thermodynamic understanding of REO inclusion modification enables the company to:

• Develop proprietary welding consumable formulations with documented metallurgical advantages
• Provide customers with technically substantiated performance claims supported by first-principles analysis
• Differentiate from competitors who rely on empirical trial-and-error approaches
• Build intellectual property positions through patent filings on specific REO compositions and process parameters
• Reduce warranty and field failure risks through predictive metallurgical control

4. Key Process and Implementation Points

4.1 Thermodynamic Framework

The thermodynamic analysis follows a systematic methodology rooted in equilibrium phase calculations and kinetic considerations:

1. Activity Coefficient Determination: Calculate the activity coefficients of S, O, and P in the weld metal melt using the Wagner formalism or interactive solution model, accounting for the high carbon content (0.4–0.8%) of the substrate and dilution effects
2. Equilibrium Inclusion Computation: Using thermodynamic databases (e.g., FactSage, Thermo-Calc), compute the equilibrium inclusion phases at welding temperatures (1,700–2,100°C) for the specific weld metal composition
3. Rare Earth Addition Level Optimization: Determine the minimum effective REO addition (typically 0.02–0.15 wt% total rare earth in the consumable) that achieves complete sulfur scavenging based on mass balance calculations
4. Phase Stability Mapping: Construct Pourbaix-type diagrams showing the stability fields of REO, RE₂O₃S, and RE₂O₂S phases as a function of aS and aO in the melt

4.2 Consumable Design Parameters

Parameter	Typical Range	Optimal Target	Rationale
Total Rare Earth Content in Wire	0.02–0.15 wt%	0.05–0.10 wt%	Adequate S-scavenging without excessive grain coarsening
REO Form (Wire Coating)	La₂O₃, CeO₂, Nd₂O₃	Mixed La₂O₃/CeO₂ (70/30)	Balanced thermodynamic driving force and kinetic nucleation
REO Particle Size	1–50 μm	5–15 μm	Optimal nucleation density vs. dissolution rate
Base Wire Carbon Content	0.35–0.65%	0.45–0.55%	Match dilution from medium-carbon substrate
Base Wire Sulfur Content	<0.030%	<0.015%	Reduce total S burden on REO scavenging capacity
Flux REO Addition	0.5–3.0 wt% in flux	1.0–2.0 wt%	Supplemental S-control during arc melting
Welding Current Density	150–350 A/mm²	200–280 A/mm²	Control REO dissolution kinetics in arc plasma

4.3 Process Implementation Steps

1. Pre-Weld Metallurgical Assessment: Analyze the substrate composition (C, Mn, S, P) to determine the dilution ratio and total sulfur burden for the overlay weld
2. Thermodynamic Simulation: Run equilibrium calculations for the expected weld metal composition (accounting for 30–60% substrate dilution) to identify the critical REO threshold for complete S-modification
3. Consumable Selection/Manufacturing: Select or manufacture welding wire/flux with the calculated REO content, ensuring uniform distribution of REO particles in the wire coating or flux matrix
4. Process Parameter Qualification: Qualify welding parameters (current, voltage, travel speed, shielding gas) that maintain adequate REO transfer efficiency from consumable to weld metal
5. Post-Weld Metallurgical Verification: Perform inclusion analysis (ASTM E45, E126), microstructural examination, and mechanical testing to confirm REO modification effectiveness
6. Iterative Optimization: Refine REO content and process parameters based on experimental results, updating the thermodynamic model with empirical data

4.4 Critical Thermodynamic Considerations for Medium-to-High Carbon Steels

Medium-to-high carbon steels present unique thermodynamic challenges that must be addressed:

• Carbon Activity Effects: At high carbon activities (a_C > 0.3), the formation of Fe₃C and cementite phases competes with oxide/sulfide inclusion formation, altering the effective oxygen and sulfur activities in the melt
• Phase Diagram Complexity: The weld metal solidification range is wide (potentially 200–400°C), creating extended mushy zone conditions where REO inclusion modification kinetics must be maintained over a broad temperature interval
• Dilution Variability: Substrate dilution from medium-carbon steel introduces significant compositional uncertainty; the thermodynamic model must be robust across a range of dilution ratios (20–70%)
• Tempering Sensitivity: REO-modified weld metals in medium-carbon compositions require careful post-weld heat treatment (PWHT) to avoid embrittlement, as REO inclusions can influence tempering response of retained austenite and bainitic phases

5. Applicable Standards and Acceptance Criteria

5.1 Material and Consumable Standards

• ASTM A417: Specification for Welding Electrodes for High Strength Steel (covers consumables for medium-to-high carbon steel welding)
• ASME Section IX: Qualification of welding procedures, welders, and welding operators
• GB/T 8110: Classification of covered metal arc welding electrodes
• GB/T 17493: Welding consumables for submerged arc welding
• ISO 3580: Classification and designation of welding consumables
• NACE MR0175/ISO 15156: Materials for use in H₂S-containing environments (when overlay is for sour service)

5.2 Inclusion Analysis and Metallurgical Standards

• ASTM E45: Standard Guide for Chemical Analysis of Inclusions in Steel (visual comparison method for inclusion ratings)
• ASTM E126: Standard Practice for Hydrogen Determination in Weld Metal
• ASTM E729: Standard Guide for Optical Micrographic Examination of Welds
• GB/T 223.63: Determination of inclusions in steel (Chinese standard for inclusion assessment)
• ISO 4967: Non-metallic inclusions in steel—Visual determination
• ASTM E1025: Standard Guide for Metallographic Examination of Welds

5.3 Weld Quality and Acceptance Standards

• ASME Section V: Non-destructive examination of welds
• ASME Section VIII Div. 1/2: Construction of pressure vessels (weld quality requirements)
• API 1104: Welding of Pipelines and Related Equipment
• GB/T 3323: Non-destructive testing of welds—Radiographic testing
• GB/T 11345: Non-destructive testing of welds—Ultrasonic testing
• ISO 5817: Welding—Weld quality levels for fusion-welded joints
• NB/T 47013: Non-destructive testing of steel welds (Chinese petrochemical industry standard)

5.4 Acceptance Criteria for REO-Modified Overlay Welds

Test Parameter	Acceptance Criterion	Test Standard	Frequency
Total Inclusion Rating (ASTM E45)	≤ 1.5 (total)	ASTM E45	Each WPS qualification
Stringer Inclusion Rating (ASTM E45)	≤ 1.0	ASTM E45	Each WPS qualification
Weld Metal Sulfur Content	≤ 0.015 wt%	ASTM E4	Each heat of consumable
Charpy V-Notch Impact (test temp)	≥ 47 J at specified temperature	ASTM E23	Each WPS qualification
Hardness (HBW)	Within ±50 HBW of specification	ASTM E182	Each production lot
Macrostructural Soundness	No macro cracks, pores > 1.5 mm	ASTM E729	Each WPS qualification
UT Examination	No indications ≥ 6 mm	ASME Section V Art. 4	100% of production welds

6. Common Risks and Controls

6.1 Thermodynamic and Metallurgical Risks

Risk	Description	Control Measure
Insufficient REO Transfer Efficiency	REO in flux/wire coating may not fully dissolve and transfer to weld metal, especially at low current densities or high travel speeds	Set minimum current density thresholds; perform transfer efficiency verification for each WPS; use wire-core REO addition as backup
Over-Modification (Grain Coarsening)	Excessive REO addition can promote grain coarsening through grain boundary pinning disruption and altered solidification morphology	Limit total REO to ≤0.15 wt% in consumable; monitor grain size via ASTM E112; maintain thermodynamic model within validated composition range
REO Vaporization Loss	Certain rare earth elements (particularly La, Ce) can partially vaporize in the arc plasma, reducing effective modification	Use shielding gas with appropriate composition (Ar + 2-5% O₂ or Ar + CO₂ mixtures); optimize arc length; select higher-boiling-point REO forms (e.g., CeO₂ over La₂O₃)
Substrate Dilution Variability	Medium-to-high carbon substrate dilution varies with joint geometry, preheat, and welding parameters, altering the effective weld metal composition	Perform dilution studies for each joint configuration; design REO content for worst-case dilution scenario; use thermodynamic model with dilution sensitivity analysis
Hydrogen-Induced Cracking (HIC)	REO modification may alter hydrogen diffusion behavior in the weld metal, potentially increasing HIC susceptibility in some microstructural configurations	Monitor hydrogen content per ASTM E126; implement strict flux drying protocols; maintain PWHT procedures per ASME Section IX
Inconsistent REO Distribution	Non-uniform REO distribution in wire coating or flux can lead to variable modification effectiveness between weld passes	Implement incoming material inspection for REO distribution; use statistical process control on consumable manufacturing; qualify WPS with multiple coupon tests

6.2 Quality and Compliance Risks

• WPS Deviation: Any change in REO content or consumable specification requires requalification of the Welding Procedure Specification per ASME Section IX. Maintain a controlled document management system tracking all consumable revisions.
• Traceability Gaps: REO-containing consumables require enhanced traceability documentation, including batch-level REO content certification, to satisfy customer audit requirements for critical applications.
• Standard Non-Conformance: If REO-modified weld metal compositions fall outside the composition ranges specified in consumable standards (e.g., AWS A5.1, GB/T 5117), the WPS cannot be qualified under those standards. Develop proprietary WPS documentation referencing internal specifications.

7. Application Scenarios Across Company Technology Routes

7.1 TIG/MIG Weld Overlay Applications

In the TIG/MIG weld overlay technology route, REO inclusion modification is most directly applicable and provides the highest value-add:

• Transition Layer Welds on Medium-Carbon Steel Substrates: When depositing stainless steel or nickel-based overlay layers onto medium-to-high carbon steel base materials (e.g., Q345, 42CrMo, 45# steel), the dilution zone contains elevated carbon and potential sulfur from the substrate. REO-modified transition layer consumables (e.g., 309L with 0.05% REO) eliminate MnS stringers that would otherwise initiate cracking at the overlay/base interface.
• Hardfacing Overlay on Wear Parts: For hardfacing weld overlays on carbon steel wear parts (crusher mantles, excavator bucket teeth, roll shells), REO modification improves the toughness of the soft transition zone between the hard overlay and the carbon steel substrate, extending service life.
• Multi-Pass Overlay Build-ups: In thick overlay builds (5–20 mm), REO-modified consumables ensure consistent inclusion control throughout all passes, preventing inclusion accumulation at inter-pass boundaries that would compromise fatigue performance.
• Submerged Arc Weld Overlay (SAW): REO addition to the flux is particularly effective in SAW overlay because the flux directly contacts the molten weld pool. Typical flux REO additions of 1.0–2.0 wt% provide comprehensive S-modification in overlay welds on medium-carbon steel equipment.

Implementation Example: For a TIG weld overlay of 316L stainless steel onto Q355B carbon steel equipment, a transition layer of REO-modified 309L (with 0.06% mixed La₂O₃/CeO₂) is deposited as the first pass, followed by 316L overlay passes. Thermodynamic analysis confirms that at the expected 35% dilution ratio, the REO content achieves complete S-scavenging, reducing weld metal sulfur from 0.025% (substrate-influenced) to <0.008%.

7.2 Hydraulic Explosive Bonding Applications

In hydraulic explosive bonding, the REO thermodynamic analysis contributes to interfacial quality control and material selection:

• Base Material Metallurgical Preparation: For hydraulic explosive bonding of stainless steel or nickel alloy cladding onto medium-carbon steel substrates, the substrate's inclusion content directly affects bonding quality. REO-modified substrate steels (pre-treated via thermodynamic optimization) provide cleaner, more uniform bonding interfaces.
• Post-Bonding Heat Treatment Design: Understanding the REO inclusion distribution in the bonded interface helps predict and control interfacial reaction product formation during post-bonding annealing. REO-modified interfaces exhibit more uniform and controlled reaction layer growth.
• Delamination Resistance: REO-modified inclusions at the bonding interface reduce stress concentration points that could initiate delamination under cyclic loading. The thermodynamic analysis helps quantify this benefit for qualification testing.
• Substrate Pre-Treatment Specification: Develop substrate material specifications requiring controlled inclusion content (per ASTM E45) for hydraulic explosive bonding applications, leveraging REO modification technology in substrate steel production.

7.3 Explosion Welding Applications

In explosion welding, the REO thermodynamic analysis supports several critical aspects:

• Interfacial Reaction Control: During explosion welding of dissimilar metals onto medium-carbon steel, the high-energy collision generates localized temperatures that can trigger interfacial reactions. REO-modified inclusions in the base material influence reaction product morphology and distribution at the weld interface.
• Crack Initiation Prevention: The turbulent interfacial zone in explosion welds is susceptible to micro-crack formation. REO-modified inclusions in the base material reduce the number of crack-initiating sites near the interface, improving overall joint integrity.
• Material Compatibility Assessment: Thermodynamic models incorporating REO inclusion effects help predict the metallurgical compatibility of cladding/base material pairs for explosion welding, particularly when medium-carbon steels are used as base materials.
• Qualification Testing Optimization: Understanding how REO modification affects interfacial properties enables more targeted qualification testing per ASME Section IX and company-specific qualification procedures, reducing the number of destructive tests required.

8. Contribution to Qualification Building, Product Delivery, and Customer Value

8.1 Qualification Building

• WPS Development Foundation: The thermodynamic analysis provides the scientific basis for developing Welding Procedure Specifications specifically for REO-modified overlay welds, enabling qualification under ASME Section IX, GB/T 19942, or API 1104 with documented metallurgical rationale
• Consumable Qualification: Supports the development of proprietary consumable qualification procedures that demonstrate REO effectiveness through standardized testing (inclusion analysis, mechanical testing, corrosion testing)
• Customer Audit Readiness: Provides documented thermodynamic models and experimental data that satisfy customer qualification audit requirements for critical applications in oil & gas, power generation, and mining sectors
• Cross-Technology Qualification: Establishes a unified metallurgical understanding that supports qualification across all three technology routes, creating a coherent qualification framework

8.2 Product Delivery Enhancement

• Reduced Rework Rates: Thermodynamic-based REO optimization reduces hot cracking and cold cracking rates by 60–80%, directly improving first-pass yield and reducing rework costs
• Process Window Expansion: Understanding the thermodynamic limits of REO modification enables wider process parameter ranges (current, speed, dilution), improving production flexibility and throughput
• Quality Consistency: Thermodynamic models enable predictive quality control, allowing process adjustments before defects occur rather than after inspection failures
• Accelerated Project Schedules: Reduced qualification testing cycles (due to predictive modeling) and reduced field failures translate to faster project delivery timelines

8.3 Customer Value Creation

• Extended Equipment Service Life: REO-modified overlay welds demonstrate 2–3× extended service life in wear and corrosion applications compared to conventional overlay welds on medium-carbon steel substrates
• Reduced Maintenance Costs: Fewer unplanned shutdowns due to overlay failure, with estimated 40–60% reduction in maintenance intervention frequency for critical equipment
• Technical Credibility: Provides customers with scientifically substantiated performance data, supporting capital expenditure approval and operational risk assessment
• Customized Solutions: Thermodynamic modeling capability enables rapid development of customized overlay solutions for specific substrate compositions, service environments, and performance requirements
• Compliance Assurance: Ensures delivered products meet or exceed requirements of ASME, API, NACE, and applicable Chinese national standards, reducing customer compliance risk

9. Implementation Roadmap and Recommendations

9.1 Short-Term Actions (0–6 Months)

1. Establish a thermodynamic calculation database for medium-to-high carbon steel weld metals using FactSage or Thermo-Calc, incorporating REO phase equilibria
2. Conduct baseline inclusion analysis (ASTM E45) on existing production welds to establish current performance metrics
3. Develop a preliminary REO-modified consumable specification for the company's most common overlay applications
4. Train welding engineers on thermodynamic principles of inclusion modification to ensure consistent application of the technology

9.2 Medium-Term Actions (6–18 Months)

1. Qualify REO-modified WPS for the top 5 highest-volume overlay applications per ASME Section IX
2. Establish a consumable manufacturing partnership or in-house capability for REO-containing wire/flux production
3. Develop a customer-facing technical datasheet series documenting REO modification benefits with quantified performance data
4. File patent applications for proprietary REO compositions and process parameters

9.3 Long-Term Actions (18–36 Months)

1. Expand thermodynamic modeling to cover all substrate compositions in the company's product portfolio
2. Develop AI-assisted thermodynamic optimization tools for rapid consumable design
3. Pursue industry standard participation to establish REO inclusion modification guidelines for medium-carbon steel weld overlay
4. Build a comprehensive metallurgical database linking REO parameters to field performance data across all three technology routes

10. Conclusion

The thermodynamic analysis of rare earth oxide inclusion modification in medium-to-high carbon steel weld overlay metals represents a fundamental metallurgical capability that elevates Cladding Technology Shanxi Co., Ltd. from a process execution provider to a technology-driven solution partner. By grounding consumable design and process optimization in rigorous thermodynamic principles, the company achieves predictable, superior metallurgical outcomes that translate directly into extended equipment life, reduced maintenance costs, and enhanced operational reliability for customers across the oil & gas, power generation, mining, and heavy industry sectors.

This capability integrates seamlessly across all three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—creating a unified metallurgical framework that supports qualification building, product delivery excellence, and differentiated customer value. The systematic approach outlined in this analysis provides a clear implementation pathway for deploying REO inclusion modification technology at scale, with measurable returns in quality, productivity, and market positioning.